Contrary to conventional memory devices, which use an electric charge to store data, a magnetoresistive random access memory (“MRAM”) device stores data magnetically. The advantages of storing data magnetically include non-volatility, high-speed operation, and durability. Although an MRAM device has these advantages over conventional memory devices, its commercial viability still depends on whether technologies can continue to be developed to improve its power consumption, error rate, and stability of the memory state.
To store and retrieve data magnetically, an MRAM device is constructed to have a magnetic moment vector that switches between two stable orientations. Each stable orientation corresponds to one of two possible memory states of an MRAM device, and has a different electrical resistance. During a write operation, a magnetic field is applied to switch the magnetic moment vector to the desirable orientation. To read the stored data, a current is passed through the MRAM device to sense the electrical resistance and determine the corresponding memory state.
Several prior references have already disclosed improvements for the MRAM device. An improved method for writing to an MRAM device known as “toggle-mode” was developed and disclosed by Savtchenko et al. in U.S. Pat. No. 6,545,906. The toggle-mode write operation improves the stability of the memory state and the selectability of the write operation by using a rotating magnetic field to write to an MRAM device. Moreover, toggle-mode MRAM devices with improved power consumption are disclosed in U.S. Pat. No. 6,633,498 and U.S. Pat. No. 6,515,341. A disclosed device comprises an extra bias field that acts on the magnetic material in the MRAM device and reduces the power required to switch the orientation of the magnetic moment vector.
Despite the improvement in stability of memory state and power consumption, a new problem arises due to the use of an extra bias field in MRAM devices. The bias field not only reduces the switching field but also affects the rotating magnetic field during write operations, and this greatly increases the error rate. Thus, there exists a need for a toggle-mode MRAM device with reduced switching field and low error rate during write operations.